Ferritic stainless steel sheet for fuel tank and fuel pipe and method for making the same

ABSTRACT

A ferritic stainless steel sheet for fuel tanks and fuel pipes comprises, by mass percent, about  0.1 % or less of C; about  1.0 % or less of Si; about  1.5 % or less of Mn; about  0.06 % or less of P; about  0.03 % or less of S; about  1.0 % or less of Al; about 11% to about 20% Cr; about 2.0% or less of Ni; about 0.5% to about 3.0% Mo; about 0.02% to about 1.0% V; about 0.04% or less of N; at least one of about 0.01% to about 0.8% Nb and about 0.01% to about 1.0% Ti; and the balance being Fe and incidental impurities. The ferritic stainless steel sheet is produced by rough-rolling a slab having the above composition; hot-rolling the rough-rolled sheet under a linear pressure of at least about 3.5 MN/m at a final pass in the finish rolling; cold-rolling the hot-rolled sheet at a gross reduction rate of at least about 75%; and annealing the cold-rolled sheet. The cold-rolling step includes one rolling stage or at least two rolling stages including intermediate annealing.

BACKGROUND

[0001] 1. Field of the Invention

[0002] This invention relates to ferritic stainless steel sheetssuitable for containers and piping elements for organic fuels such asgasoline, methanol and the like. In particular, the invention relates toa ferritic stainless steel sheet which can be readily shaped into fueltanks and fuel pipes and which is resistant to organic fuels,particularly deteriorated gasoline containing organic acids produced inthe ambient environment. The invention also relates to a method formaking the ferritic stainless steel sheet.

[0003] 2. Description of the Related Art

[0004] Automobile fuel tanks are generally manufactured by platingsurfaces of a soft steel sheet with a lead alloy and shaping and weldingthe terne coated steel sheet. The continued use of lead-containingmaterials, however, tends to be severely limited with the increasingsensitivity to environmental issues.

[0005] Several substitutes for the terne coated steel sheet have beendeveloped. Unfortunately, the substitutes have the following problems.Al—Si plating materials as lead-free plating materials are unreliable inweldability and long-term corrosion resistance and, thus, are used onlyin restricted fields. Although resinous materials have been tried foruses in fuel tanks, industrial use of the resinous materials which areinevitably permeable to fuel is limited under circumstances such asregulations against fuel transpiration and recycling. Also, the use ofaustenitic stainless steels, which requires no lining treatments, hasbeen attempted. Although the austenitic stainless steels exhibitsuperior processability and higher corrosion resistance compared withthe ferritic stainless steels, the austenitic stainless steels areexpensive for fuel tanks and have the possibility of stress corrosioncracking (SCC). Thus, the austenitic stainless steels have not yet beenused in practice.

[0006] In contrast, the ferritic stainless steels not containing nickelare advantageous in material costs compared with the austeniticstainless steels, but do not exhibit satisfactory corrosion resistanceto so-called “deteriorated gasoline” containing organic acids such asformic acid and acetic acid which are formed in the ambient environment.Furthermore, the ferritic stainless steels do not exhibit sufficientprocessability to deep drawing for forming fuel tanks having complicatedshapes and to expanding and bending of the pipes for forming expandedfuel pipes and bent fuel pipes.

[0007] Japanese Unexamined Patent Publication Nos. 6-136485 and 6-158221disclose double-layer steel sheets each including a corrosion-resistantsteel layer and a low-carbon or ultra-low-carbon steel layer havingexcellent processability to achieve both corrosion resistance andprocessability. However, the double-layer steel sheets exhibit lessadaptability to mass production.

SUMMARY OF THE INVENTION

[0008] The invention provides a ferritic stainless steel sheet whichexhibits superior processability and high corrosion resistance todeteriorated gasoline and is useful for automobile fuel tanks and fuelpipes. In particular, the ferritic stainless steel of the invention hasa thickness in the range of about 0.4 to about 1.0 mm and superior deepdrawing processability, namely, an r-value of at least about 1.50 andpreferably at least about 1.90.

[0009] The r-value in the invention represents a mean plastic strainratio determined by equation (1) according to Japanese IndustrialStandard (JIS) Z2254: $r = \frac{r_{0} + {2\quad r_{45}} + r_{90}}{4}$

[0010] wherein,

[0011] r₀ is a plastic strain ratio measured using a test piece which issampled in parallel to the rolling direction of the sheet;

[0012] r₄₅ is a plastic strain ratio measured using a test piece whichis sampled at 45° to the rolling direction of the sheet; and

[0013] r₉₀ is a plastic strain ratio measured using a test piece whichis sampled at 90° to the rolling direction of the sheet.

[0014] An r-value of less than about 1.50 precludes deep drawing into acomplicated fuel tank shape and bending into a complicated bent pipeshape and exhibits high impact brittleness (secondary processingbrittleness) even if the sheet is capable of processing.

[0015] The invention also provides a ferritic stainless steel having asurface ridging height of about 50 μm or less at 25% deformation inuniaxial stretching. Ridges formed during processing of steel sheets forautomobile fuel tanks are not necessarily so small because these tanksare produced by press forming of the sheet. According to ourinvestigations, however, ridges cause cracking of the sheet duringsevere press forming processes which are used in the production of fueltanks. Hence, the ridging height must be small. The ridges generated inthe sheeting process vary the state of contact of the unprocessed steelsheet piece with the press die and results in “gnawing” or “galling” dueto a local deficiency of lubricant oil film. The gnawing also causescracking along the ridges.

[0016] According to our further investigations, a steel sheet exhibitingsuperior press formability suitable for processing of fuel tanks havingcomplicated shapes has a surface ridging height of about 50 μm or lessat a 25% deformation in uniaxial stretching. Herein, the ridges on thesteel sheet generated during processing are evaluated by the height ofthe ridges in a direction perpendicular to the stretching direction whenthe steel is stretched in the rolling direction.

[0017] The invention also solves a problem in the art known in the caseof severe forming of a ferritic stainless steel into fuel tanks and fuelpipes and in the case of lubricant-free press forming. That is, theinvention provides a ferritic stainless steel by a lubricant-freeprocess exhibiting superior deep drawability and requires no lubricationsteps for treating the sheet with lubricant oil.

[0018] We discovered that a predetermined amount of a lubricant coatprimarily containing an acrylic resin which is applied on the surfacesof a ferritic stainless steel sheet decreases the dynamic frictioncoefficient between the steel sheet and the press die, thus preventing“gnawing” and being capable of processing into articles having furthercomplicated shapes.

[0019] We intensively investigated the effects of the composition offerritic stainless steel sheets and the method for making the same onthe corrosion resistance in deteriorated gasoline and the r-value of theferritic stainless steel sheet and found that the corrosion resistanceto the deteriorated gasoline is remarkably improved by addingappropriate amounts of Mo and V to the steel sheets.

[0020] Since the addition of Mo precludes processability, we furtherinvestigated the r-value as a reference of processability ofMo-containing steel sheets and found that a high r-value is achieved bya specified method.

[0021] Furthermore, we found that optimized annealing conditions forhot-rolled ferritic stainless steel sheets minimize the ridging height,provide superior press formability, and that the application of alubricant coat on the steel sheet surfaces improves sliding performancein forming, decreases the dynamic friction coefficient between the steelsheet and the press die, and facilitates forming of articles havingfurther complicated shapes.

[0022] According to an aspect of the invention, a ferritic stainlesssteel sheet for fuel tanks and fuel pipes comprises, by mass percent,about 0.1% or less of C; about 1.0% or less of Si; about 1.5% or less ofMn; about 0.06% or less of P; about 0.03% or less of S; about 1.0% orless of Al; about 11% to about 20% Cr; about 2.0% or less of Ni; about0.5% to about 3.0% Mo; about 0.02% to about 1.0% V; about 0.04% or lessof N; at least one of about 0.01% to about 0.8% Nb and about 0.01% toabout 1.0% Ti; and the balance being Fe and incidental impurities.

[0023] Preferably, the ferritic stainless steel sheet has a ridgingheight of about 50 μm or less at a 25% deformation in uniaxialstretching.

[0024] Preferably, a lubricant coat comprising an acrylic resin, calciumstearate, and polyethylene wax is coated by baking on the surfaces ofthe ferritic stainless steel sheet in a coating amount of about 0.5 g/m²to 4.0 g/m².

[0025] According to another aspect of the invention, a method for makinga ferritic stainless steel sheet for fuel tanks and fuel pipes,comprises the steps of rough-rolling a slab comprising, by mass percent,about 0.1% or less of C, about 1.0% or less of Si, about 1.5% or less ofMn, about 0.06% or less of P, about 0.03% or less of S, about 1.0% orless of Al, about 11% to about 20% Cr, about 2.0% or less of Ni, about0.5% to about 3.0% Mo, about 0.02% to about 1.0% V, about 0.04% or lessof N, at least one of about 0.01% to about 0.8% Nb and about 0.01% toabout 1.0% Ti, and the balance being Fe and incidental impurities;hot-rolling the rough-rolled sheet under a linear pressure of at leastabout 3.5 MN/m at a final pass in the finish rolling; cold-rolling thehot-rolled sheet at a gross reduction rate of at least about 75%, thecold-rolling step including one rolling stage or at least two rollingstages including intermediate annealing; and annealing the cold-rolledsheet.

[0026] Preferably, the hot-rolled sheet is subjected to hot-rolled sheetannealing according to the following equations:

900≦T+20t≦1,150 and t≦10

[0027] wherein T is the annealing temperature (° C) and t is the holdingtime (minutes).

[0028] Preferably, a lubricant coat comprising an acrylic resin, calciumstearate, and polyethylene wax is coated by baking on the surfaces ofthe hot-rolled or annealed hot-rolled sheet in a coating amount of about0.5 g/m² to about 4.0 g/m².

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a graph illustrating the effects of the Mo and Vcontents in ferritic stainless steel sheets on the corrosion resistancein the deteriorated gasoline;

[0030]FIG. 2 is a graph illustrating the effects of the linear pressureat the final pass in the finish rolling and the gross cold-rollingreduction rate on the r-value of the final product; and

[0031]FIG. 3 is a graph illustrating the effects of the hot-rolled sheetannealing condition on the ridging height.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0032] Reasons for limitation of the composition and process conditionsof the ferritic stainless steel sheet according to the invention willnow be described. The content of each component is represented by masspercent (hereinafter merely referred to as percent or %).

[0033] C: about 0.1% or less

[0034] Although a required amount of carbon (C) is added to strengthengrain boundaries and to enhance brittle resistance to secondaryprocessing, excess carbon precipitates at grain boundaries as carbideswhich adversely affect brittle resistance to secondary processing andcorrosion resistance at grain boundaries. Since these adverse affectsare noticeable at a C content exceeding about 0.1%, the C content islimited to be about 0.1% or less. The C content is preferably in therange of more than about 0.002% to about 0.008% in view of animprovement in brittle resistance in secondary processing.

[0035] Si: about 1.0% or less

[0036] Silicon (Si) contributes to improved oxidation and corrosionresistance and, thus, improved corrosion resistance on the outer andinner surfaces of a fuel tank. In order to achieve such effects, the Sicontent is preferably about 0.2% or more. However, a Si contentexceeding about 1.0% causes the embrittlement of the steel sheet and thedeterioration of brittle resistance in secondary processing at the weld.Thus, the Si content is about 1.0% or less and preferably about 0.75% orless.

[0037] Mn: about 1.5% or less

[0038] Manganese (Mn) improves oxidation resistance. Although about 0.5%or more of Mn is preferably used to achieve such an effect, an excessamount of Mn causes the deterioration of toughness of the steel sheetand the deterioration of brittle resistance in the secondary processingat the weld. Thus, the Mn content is about 1.5% or less and preferablyabout 1.30% or less.

[0039] P: about 0.06% or less

[0040] Phosphorus (P) readily precipitating at grain boundariesdecreases the strength at the grain boundaries after severe processingsuch as deep drawing for making fuel tanks. Thus, the P content ispreferably as low as possible to improve brittle resistance in secondaryprocessing (resistance to cracking by slight impact after severeprocessing). Since a significantly low P content results in an increasein production cost of steel-making process, the P content is about 0.06%or less and more preferably about 0.03% or less.

[0041] S: about 0.03% or less

[0042] Although sulfur (S) precludes corrosion resistance of thestainless steel, about 0.03% is allowable as the upper limit in view ofdesulflrization cost in of steel-making process. Preferably, the Scontent is about 0.01% or less which can be fixed by Mn and Ti:

[0043] Al: about 1.0% or less

[0044] Although aluminum (Al) is an essential element as a deoxidizer inthe steel-making process, an excess amount of aluminum causesdeterioration of surface appearance and corrosion resistance due toinclusions. Thus, the Al content is limited to be about 1.0% or less andpreferably about 0.50% or less.

[0045] Cr: about 11% to 6.20%

[0046] At least about 11% chromium (Cr) must be contained in the steelto achieve sufficient brittle and corrosion resistance. On the otherhand, a Cr content exceeding about 20% results in the deterioration ofprocessability due to increased strength and decreased ductility even ifthe revalue is high. Thus, the Cr content is in the range of about 11%to about 20%. Preferably, the Cr content is about 14% or more and morepreferably in the range of about 14% to about about 18%, in view ofcorrosion resistance at the weld.

[0047] Ni: about 2.0% or less

[0048] At least about 0.2% nickel (Ni) is preferably contained toimprove the corrosion resistance of the stainless steel. An amountexceeding about 2.0% nickel causes hardening of the steel and stresscorrosion cracking due to the formation of an austenite phase. Thus, theNi content is about 2.0% or less and preferably in the range of about0.2% to about 0.8%.

[0049] Mo: about 0.5% to 3.0%

[0050] Molybdenum (Mo), as well as vanadium (V), is effective in animprovement in corrosion resistance to deteriorated gasoline. Atleastabout 0.5% Mo is required to achieve superior corrosion resistance todeteriorated gasoline. However, a Mo content exceeding about 3.0%results in deterioration of processability due to precipitation formedduring annealing. Thus, the Mo content is in the range of about 0.5% toabout 3.0% and preferably about 0.7% to about 1.6%.

[0051] V: about 0.02% to 1.0%

[0052] Vanadium (V) is effective in an improvement in corrosionresistance to deteriorated gasoline by a combination with molybdenum(Mo). Such an improvement is observed at a V content of at least about0.02%. However, a V content exceeding about 1.0% results in thedeterioration of processability due to precipitation during annealing.Thus, the V content is in the range of about 0.02% to about 1.0% andpreferably about 0.05% to about 0.3%.

[0053] The relationships between the Mo and V contents and the corrosionresistance to deteriorated gasoline will now be described. FIG. 1 is agraph illustrating the relationships between the Mo and V contents inferritic stainless steel sheets and the corrosion resistance. Theferritic stainless steel sheets contains about 0.003% to about 0.005% C,about 0.07% to about 0.13% Si, about 0.15% to about 0.35% Mn, about0.02% to about 0.06% P, about 0.01% to about 0.03% S, about 14.5% toabout 18.2% Cr, about 0.2% to about 1.0% Ni, about 0.02% to about 0.04%Al, about 0.001% to about 0.45% Nb, about 0.3% to about 0.5% Ti, andabout 0.004% to about 0.011% N, and the corrosion resistance is measuredin a deteriorated gasoline containing 800 ppm. of formic acid for 120hours. In the graph, the symbol ◯ represents that the appearance afterthe corrosion resistance test in the deteriorated gasoline does notchange, and the symbol  represents that the surface red rust isobserved.

[0054]FIG. 1 shows that samples containing both Mo and V and having a Mocontent of about 0.5% or more and a V content of about 0.02% or moreexhibit high corrosion resistance in the deteriorated gasoline.

[0055] N: about 0.04% or less

[0056] Although nitrogen (N) strengthens grain boundaries which improvesbrittle resistance in secondary processing for making tanks and thelike, an excess amount of nitrogen precipitates at the grain boundariesas nitrides which adversely affects corrosion resistance. Thus, the Ncontent is about 0.04% or less and preferably about 0.020% or less.

[0057] Nb: about 0.01% to about 0.8% and Ti: about 0.01% to about 1.0%

[0058] Niobium (Nb) and titanium (Ti) fix carbon and nitrogen in asolid-solution state as compounds to increase the revalue. The contentof each element to fix carbon and nitrogen is about 0.01% or more. Theseelements may be contained alone or in combination. A Nb contentexceeding about 0.8% causes remarkable deterioration of toughness, and aTi content exceeding about 1.0% causes deterioration of the surfaceappearance and toughness. Preferably, the Nb content is in the range ofabout 0.05% to about 0.4% and the Ti content is in the range of about0.05% to about 0.40%.

[0059] The ferritic stainless steel sheet of the invention may furthercontain about 0.3% or less of cobalt (Co) and about 0.01% or less ofboron (B) to improve brittle resistance in secondary processing.Moreover, the ferritic stainless steel sheet may contain the followingincidental impurities: about 0.5% or less of zirconium (Zr), about 0.1%or less of calcium (Ca), about 0.3% or less of tantalum (Ta), about 0.3%or less of tungsten (W), abouit 1% or less of copper (Cu), and about0.3% or less of tin (Sn), as long as the steel sheet exhibits theabove-described advantages.

[0060] The ferritic stainless steel sheet according to the invention maybe produced by a known method which is generally employed in productionof ferritic stainless steel sheets. However, conditions for hot rollingand cold rolling are partly changed, as described below. In steelmalking, preferably, steel containing the above essential components andauxiliary components added according to demand is produced in aconverter or electric furnace and the steel is subjected to secondaryrefinement by vacuum oxygen decarbonization (VOD). The molten steel maybe subjected to any known casting process and preferably a continuouscasting process in view of productivity and quality. The steel materialobtained by the continuous casting process is heated to a temperaturebetween about 1,000° C. and about 1,250° C. and hot-rolled to form ahot-rolled steel sheet having a desired thickness.

[0061] The linear pressure at the final pass in the hot rolling is atleast about 3.5 MN/m to continuously produce a steel sheet having a highrevalue. The linear pressure represents a pressure during rollingdivided by the sheet width. A larger linear pressure is considered tocontinuously obtain a high r-value because strain is accumulated in thesteel sheet. A large linear pressure is achieved by any combination of adecrease in hot rolling temperature, high-alloy formulation, an increasein hot rolling speed, and an increase in roller diameter.

[0062] The resulting hot-rolled sheet is, if necessary and preferably,subjected to continuous annealing (hot-rolled sheet annealing) at atemperature in the range of about 900° C. to about 1,100° C., pickling,and cold rolling to form a cold-rolled sheet. The cold rolling step mayinclude at least two cold rolling stages including an intermediateannealing for production procedure reasons, if necessary. In order toproduce a steel sheet having a high r-value, the above-described linearpressure at the final pass in the hot rolling must be secured and thegross reduction rate in the cold rolling step including one cold rollingstage or two cold rolling stages must be at least about 75% and morepreferably at least about 82%.

[0063] The cold-rolled sheet is preferably subjected to continuousannealing (cold-rolled sheet annealing) at a temperature in the range ofabout 800° C. to about 1,100° C. and pickling to form a cold-rolledannealed sheet as the final product. The cold-rolled annealed sheet maybe subjected to slight rolling to adjust the shape and quality of thesteel sheet according to the usage.

[0064]FIG. 2 is a graph illustrating the effects of the linear pressureat the final pass in the finish hot rolling of slabs and the grossreduction rate of the subsequent cold rolling on the r-value of thefinal product in which the slab contains about 0.003% to about 0.005% C,about 0.07% to about 0.13% Si, about 0.15% to about 0.35% Mn, about0.02%. to about, 0.06% P, about 0.01% to about 0.03% S, about 14.5% toabout 18.2% Cr, about 0.2% to about 1.0% Ni, about 0.5% to about 1.6%Mo, about 0.02% to about 0.43% V, about 0.02% to about 0.04% Al, about0.001% to about 0.45% Nb, about 0.3% to about 0.5% Ti, about 0.004% toabout 0.011% N, and the balance substantially being Fe.

[0065]FIG. 2 shows that a high revalue is always achieved at a linearpressure at the hot-rolling final pass of at least about 3.5 MN/m and agross cold-roling reduction rate of at least about 75% in high-alloysteels containing at least about 0.5% Mo.

[0066] The method for making the steel sheet according to the inventionwill now be described. The steel sheet according to the invention isproduced by a known method employed in production of ferritic stainlesssteel sheets, but the production conditions are partly modified. Thatis, the cold-rolled annealed steel sheet is produced through steelmaking, hot rolling, annealing, pickling, cold rolling and finishannealing.

[0067] Steel having the above composition is produced in a converter orelectric furnace and the melt subjected to secondary refinement by VOD.The molten steel may be subjected to any known casting process and,preferably, a continuous casting process in view of productivity andquality. The steel material obtained by the continuous casting processis heated to a temperature between about 1,000° C. and about 1,250° C.and hot-rolled to form a hot-rolled steel sheet having a desiredthickness.

[0068] The hot-rolled sheet is annealed. Annealing conditions areessential for continuous production of steel sheets having low ridgingheight and superior press formability. The annealing temperature T (° C)and the holding time t (minutes) are determined so as to satisfy therelationship 900≦T+20t≦1,150. Continuous heating furnaces are generallyused in industrial facilities. The holding time t is preferably about 10minutes or less in view of productivity and controllability.

[0069]FIG. 3 is a graph illustrating the effects of the hot-roled sheetannealing condition on the ridging height of a ferritic stainless steelsheet containing about 0.003% to about 0.005% C, about 0.07% to about0.13% Si, about 0.15% to about 0.35% Mn, about 0.02% to about 0.06% P,about 0.01% to about 0.03% S, about 14.5% to about 18.2% Cr, about 0.2%to about 1.0% Ni, about 0.5% to about 1.6% Mo, about 0.04% to about0.43% V, about 0.02% to about 0.04% Al, about 0.001% to about 0.45% Nb,about 0.3% to about 0.5% Ti, about 0.004% to about 0.011% N. and thebalance being Fe. FIG. 3 suggests that a combination of an annealingtemperature T and a holding time t satisfying the relationship900≦T+20t≦1,150 can achieve a ridging height of about 50 μm or less.

[0070] Cold rolling is performed at a gross rolling reduction rate ofabout 84%, a finish annealing temperature of about 900° C., and aholding time of about 60 seconds.

[0071] After annealing, the hot-rolled steel sheet is subjected topickling and cold rolling to produce a cold-rolled sheet. This coldrolling step may include two or more cold rolling stages includingintermediate annealing for production procedure reasons, if necessary.Preferably, the gross rolling reduction rate during the cold rolling isat least about 75%. The cold-rolled sheet is preferably subjected to(continuous) finish annealing at a temperature between about 800° C. andabout 1,100° C. and pickling to produce a cold-rolled annealed sheet asa final product. The cold-rolled annealed sheet may be subjected toslight rolling to adjust the shape and quality of the steel sheetaccording to usage.

[0072] In order to omit lubricant vinyl or oil in severe processing forcomplicated shapes and press forming, a lubricant coat is preferablyapplied to the surfaces of the steel sheet in a coating amount of about0.5 g/m² to about 4.0 g/m². The lubricant coat in the invention containsabout 3 to about 20 percent by volume of calcium stearate and about 3 toabout 20 percent by volume of polyethylene wax.

[0073] The applied lubricant coat improves sliding performance of thesteel sheet and facilitates deep drawing into complicated shapes.Preferably, the lubricant coat is a removable type which can be readilyremoved with alkali. If the steel sheet containing the remaininglubricant coat is subjected to spot welding or seam welding, sensitiveweld portions cause noticeable deterioration of corrosion resistance.

[0074] According to press forming testing, at least about 0.5 g/m² oflubricant coat must be applied to ensure the improvement in slidingperformance. At a coating amount exceeding about 4.0 g/m², the effect ofthe lubricant coat is no longer enhanced. Furthermore, the steel sheethaving such a high amount of lubricant coat amount is not suitable forseam welding or spot welding because the lubricant coat precludeselectrical conduction in the welding process and causes excessivesensitivity at the welding portion. The coating amount of the lubricantcoat on the steel sheet is preferably about 1.0 to about 2.5 g/m² inview of compatibility between weldability and processability. Thelubricant coat may be applied to one side or preferably two sides of thestainless steel.

[0075] The thickness of the steel sheet made by the above productionsteps is preferably at least about 0.4 mm to ensure that sufficientstrength is imparted to a tank filled with fuel. However, excessthickness results in a decrease in cold rolling reduction rate andr-value, thereby precluding press formability and pipe expansion. Hence,the maximum thickness is preferably about 1.0 mm. The resulting steelsheet according to the invention has an r-value of at least about 1.50or at least about 1.90 under optimized production conditions. Thus, thesteel sheet according to the invention exhibits high corrosionresistance and high toughness after the steel sheet is shaped into afuel tank or a pipe. Fuel pipes made of the steel sheet according to theinvention may be welded by any known welding method such as arc weldingincluding tungsten inert gas (TIG) welding, metal inert gas (MIG)welding, and ERW; electric resistance welding; and laser welding.

EXAMPLES Example 1

[0076] Steel slabs having the compositions shown in Table 1 were heatedto 1,120° C., and hot-rolled to form hot-rolled sheets having athickness in the range of 4.0 to 5.5 mm. Each hot-rolled sheet wascontinuously annealed (hot-rolled annealing) and then cold-rolled. Theresulting cold-rolled sheet was continuously annealed (cold-rolledannealing) and subjected to pickling to remove scales. Test steel sheetswere thereby prepared.

[0077] Table 2 shows process conditions, such as linear pressure of thefinal pass in the hot rolling, gross rolling reduction rate in the coldrolling, and annealing temperature.

[0078] The r-value of each test steel sheet was measured according toJIS-Z2254. The steel sheet was subjected to cylindrical deep drawing ata punch diameter of 33 mm and a blank diameter of 70 mm and cracking wasvisually observed. The deep drawn sample was immersed in deterioratedgasoline containing 1,200 ppm of formic acid and 400 ppm of acetic acidfor 5 days for corrosion testing. In “Corrosion resistance todeteriorated gasoline” in Table 2, letter “A” represents a change inweight of 0.1 g/m² or less and no red rust in appearance observation,and letter “B” represents cases other than “A”.

[0079] Table 2 also includes the results of other tests. Table 2 showsthat the steel sheets according to the invention exhibit superiorprocessability and high corrosion resistance to deteriorated gasoline.TABLE 1 Steel Composition (mass %) No. C Si Mn P S Al Cr Ni V Mo Nb Ti NRemarks 1 0.004 0.10 0.18 0.04 0.01 0.04 18.2 0.2 0.06 1.2 0.002 0.3000.010 EX 2 0.004 0.10 0.18 0.04 0.01 0.04 18.2 0.2 0.01 1.2 0.002 0.3000.010 CE 3 0.011 0.14 0.28 0.03 0.02 0.03 17.9 0.3 0.72 0.7 0.300 0.2000.010 EX 4 0.006 0.26 0.22 0.02 0.01 0.02 14.8 0.7 0.18 1.6 0.045 0.0100.007 EX 5 0.007 0.24 0.25 0.05 0.02 0.08 11.2 0.4 0.05 2.1 0.05 0.3500.009 EX 6 0.004 0.35 0.10 0.03 0.01 0.15 15.5 0.8 0.08 0.4 0.04 0.010.006 CE 7 0.015 0.45 0.40 0.04 0.02 0.02 17.3 0.4 0.52 0.8 0.004 0.0040.005 CE

[0080] TABLE 2 Gross Inter- Cold- cold mediated rolled Hot rollingHot-rolled annealing rolling annealing annealing Sheet Steel Final passlinear FDT Temp. reduction Temp. Time Temp. Time No. No. pressure (MN/m)(° C.) (° C.) Time (s) rate (%) (° C.) (s) (° C.) (s) A 1 5.8 780 980 6084 — — 920 60 B 1 3.6 800 930 150 85 900 60 960 40 C 1 4.2 820 910 10076 900 60 890 75 D 1 4.8 750 870 300 94 810 120 1000 94 E 1 4.4 810 930200 80 850 150 930 120 F 1 4.4 810 930 200 80 850 150 930 120 G 1 3.9790 910 150 82 — — 960 200 H 1 3.4 790 960 80 84 880 250 960 150 I 1 3.8820 930 80 74 880 150 980 80 J 1 3.8 830 940 120 77 920 100 950 120 K 23.2 870 980 60 84 — — 920 60 L 2 5.8 820 930 100 84 900 120 940 160 M 35.4 780 980 60 84 — — 920 60 N 4 7.1 760 1020 60 87 990 60 970 60 O 53.8 740 880 90 84 800 150 940 120 P 6 4.2 810 950 60 83 850 120 950 80 Q7 4.4 820 930 120 80 900 120 940 80 Corrosion Low resis- pressure FinalCracking tance rolling sheet during to dete- Sheet Steel reductionthick- r- deep riorated Re- No. No. rate (%) ness (mm) value drawinggasoline marks A 1 — 0.8 2.01 Not observed A EX B 1 — 0.8 1.90 Notobserved A EX C 1 — 1.0 1.61 Not observed A EX D 1 — 0.4 2.80 Notobserved A EX E 1 — 1.0 1.80 Not observed A EX F 1 3 1.0 1.81 Notobserved A EX G 1 — 1.0 1.90 Not observed A EX H 1 — 0.7 1.40 Observed ACE I 1 — 1.0 1.20 Observed A CE J 1 — 1.1 1.33 Observed A EX K 2 — 0.81.41 Observed B CE L 2 — 0.8 1.60 Not observed B CE M 3 — 0.8 1.60 Notobserved A EX N 4 — 0.6 1.91 Not observed A EX O 5 — 0.7 2.00 Notobserved A EX P 6 — 0.8 1.93 Not observed B CE Q 7 — 1.0 1.20 Observed ACE

Example 2

[0081] Steel slabs having the compositions shown in Table 3 were heatedto 1,120° C., and hot-rolled at a final hot-rolling temperature of 780°C. to form hot-rolled sheets having a thickness of 5.0 mm. Eachhot-rolled sheet was annealed under the conditions shown in Table 4,subjected to pickling for descaling, and then cold-rolled into athickness of 0.8 mm. The gross reduction rate in the, cold rolling stepwas 84%. The resulting cold-rolled sheet was finish-annealed at 900° C.or more and subjected to pickling to remove scales. Test steel sheetswere thereby prepared.

[0082] Tensile test pieces were prepared from each steel sheet such thatthe stretching direction corresponded to the rolling direction. One ofthe test pieces was deformed by 25% by uniaxial stretching. The heightof ridges generated on the surface of the deformed steel sheet wasmeasured in the direction perpendicular to the stretching direction.Another test piece was subjected to a bulging test with a 100-mmdiameter spherical punch and a commercially available lubricant oil inwhich the bulged height when a crack was formed was measured, as pressformability. Another test piece was prepared from each steel sheet andimmersed in a deteriorated gasoline containing 1,200 ppm of formic acidand 400 ppm of acetic acid for 5 days for corrosion testing. In“Corrosion resistance to deteriorated gasoline” in Table 2, letter “A”represents a change in weight of 0.1 g/m² or less and no red rust inappearance observation, and “B” represents cases other than “A”.

[0083] Table 4 also includes the results of these tests.

[0084] Table 4 shows that each sheet according to the invention has asmall ridging height and thus exhibits superior processability. TABLE 3Steel Composition (mass %) No. C Si Mn P S Al Cr Ni Mo V Nb Ti N A 0.0040.70 0.18 0.04 0.01 0.04 18.2 0.2 2.1 0.06 0.60 0.30 0.010 B 0.011 0.141.20 0.03 0.02 0.03 17.9 0.3 0.7 0.72 0.30 0.20 0.010 C 0.006 0.26 0.220.02 0.007 0.02 14.8 1.4 1.6 0.80 0.045 0.003 0.007 D 0.080 0.10 0.280.04 0.01 0.40 11.8 0.7 1.2 0.18 0.002 0.05 0.020 E 0.004 0.70 0.19 0.030.01 0.03 18.3 0.2 0.4 0.07 0.60 0.31 0.010 F 0.005 0.69 0.18 0.04 0.010.03 18.2 0.2 1.3 0.003 0.50 0.30 0.010

[0085] TABLE 4 Hot-rolled Corrosion annealing Ridging Bulged resistanceto Sheet Steel Temp. Time height height *) deteriorated No. No. (° C.)(m) (μm) (mm) Evaluation gasoline Remarks 1 A 1100 0.5 40 38 H A EX 2 A1000 0.5 38 43 H A EX 3 A 900 0.5 39 37 H A EX 4 A 750 0.5 48 33 M A EX5 B 1000 2.0 33 46 H A EX 6 B 900 2.0 32 49 H A EX 7 B 1000 3.0 29 49 HA EX 8 B 750 2.0 47 34 M A EX 9 C 850 4.0 24 51 H A EX 10 C 800 6.0 3543 H A EX 11 C 950 6.5 36 49 H A EX 12 C 1100 5.0 59 26 L A CE 13 D 8507.0 45 44 H A EX 14 D 800 8.0 42 46 H A EX 15 D 850 9.5 46 39 H A EX 16D 800 9.5 45 38 H A EX 17 D 1150 8.5 54 24 L A CE 18 D 1000 8.5 61 22 LA CE 19 E 1000 0.5 40 42 H B CE 20 F 900 0.5 38 36 H B CE

Example 3

[0086] Cold-rolled steel sheets A (thickness: 0.8 mm) shown in Table 2in EXAMPLE 1 were washed with an alkaline solution, and various amountsof lubricant coat containing an acrylic resin as a main component, 5percent by volume of calcium stearate, and 5 percent by volume ofpolyethylene wax were applied to these steel sheets. Each sheet, wasbaked at 80±5° C. for 15 seconds. The spot weldability and slidingperformance of test pieces prepared from each sheet were examined. Theresults are shown in Table 5.

[0087] In the sliding performance testing, a test piece with a length of300 mm and a width of 10 mm was disposed between flat dies with acontact area with the test piece of 200 mm² under an area pressure of 8kgf/mm² and a dynamic friction coefficient (μ) was determined by apulling-out force (F). The spot weldability was evaluated by a nuggetdiameter at the welded portion of two test pieces with a thickness of0.8 mm which were welded using a chromium-copper alloy (diameter 16 mm)and a R type electrode (radius=40 mm) at a current of 5 kA under apressure of 2 KN. A nugget diameter of 3{square root}t or less wasevaluated as unsatisfactory welding performance (B) and a nuggetdiameter exceeding 3{square root}t was evaluated as satisfactory weldingperformance (A) wherein t means the sheet thickness.

[0088] According to the results, at least about 0.5 g/m² of lubricantcoat must be applied to improve the sliding performance. However, at acoating amount exceeding about 4.0 g/m², the improvement in slidingperformance is saturated and weldability precluded due to poorelectrical conductivity during the spot welding. TABLE 5 Sliding testCoating amount (Dynamic friction Weldabiity (g/m²) coefficient: μ)(Nugget diameter) 0.2 0.265 A 0.4 0.166 A 0.5 0.102 A 0.8 0.101 A 1.50.099 A 2.2 0.097 A 2.8 0.097 A 3.8 0.098 A 4.2 0.097 B 5.0 0.097 B

[0089] As described above, the ferritic stainless steel sheet accordingto the invention exhibits superior processability and high corrosionresistance to deteriorated gasoline. Thus, containers and pipingelements produced using this steel sheet can be safely used in severeenvironments, for example, in the presence of deteriorated gasoline ormethanol.

What is claimed is:
 1. A ferritic stainless steel sheet for fuel tanksand fuel pipes comprising, by mass percent: about 0.1% or less of C;about 1.0% or less of Si; about 1.5% or less of Mn; about 0.06% or lessof P; about 0.03% or less of S; about 1.0% or less of Al; about 11% toabout 20% Cr; about 2.0% or less of Ni; about 0.5% to about 3.0% Mo;about 0.02% to about 1.0% V; about 0.04% or less of N; at least one ofabout 0.01% to about 0.8% Nb and about 0.01% to about 1.0% Ti; and thebalance being Fe and incidental impurities.
 2. The ferritic stainlesssteel sheet according to claim 1, wherein the ferritic stainless steelsheet has a ridging height of about 50 μm or less at a 25% deformationin uniaxial stretching.
 3. The ferritic stainless steel sheet accordingto claim 1, wherein a lubricant coat comprising an acrylic resin,calcium stearate, and polyethylene wax is coated and baked on at leastone surface of the ferritic stainless steel sheet in a coating amount ofabout 0.5 g/m² to about 4.0 g/m².
 4. The ferritic stainless steel sheetaccording to claim 2, wherein a lubricant coat comprising an acrylicresin, calcium stearate, and polyethylene wax is coated and baked on atleast one surface of the ferritic stainless steel sheet in a coatingamount of about 0.5 g/m² to about 4.0 g/m².
 5. A fuel tank comprisingthe ferritic stainless steel sheet according to claim
 1. 6. A fuel pipecomprising the ferritic stainless steel sheet according to claim
 1. 7.The ferritic stainless steel sheet according to claim 1, wherein theferritic stainless steel sheet has an r-value of at least about 1.5. 8.A method for making a ferritic stainless steel sheet for fuel tanks andfuel pipes, comprising the steps of: rough-rolling a slab comprising, bymass percent, about 0.1% or less of C, about 1.0% or less of Si, about1.5% or less of Mn, about 0.6% or less of P, about 0.03% or less of S,about 1.0% or less of Al, about 11% to about 20% Cr, about 2.0% or lessof Ni, about 0.5% to about 3.0% Mo, about 0.02% to about 1.0% V, about0.04% or less of N, at least one of about 0.01% to about 0.8% Nb andabout 0.01% to about 1.0% Ti, and the balance being Fe and incidentalimpurities; hot-rolling the rough-rolled sheet under a linear pressureof at least about 3.5 MN/m at a final pass in the finish rolling;cold-rolling the hot-rolled sheet at a gross reduction rate of at leastabout 75%, the cold-rolling including one rolling stage or at least tworolling stages including intermediate annealing; and annealing thecold-rolled sheet.
 9. The method according to claim 8, wherein thehot-rolled sheet is subjected to hot-rolled sheet annealing according tothe following equations, cold roiling, and finish annealing:900≦T+20t1,150 and t≦10 wherein T is annealing temperature (° C) and tis holding time (minutes).
 10. The method according to claim 8, whereina lubricant coat comprising an acrylic resin, calcium stearate, andpolyethylene wax is coated and baked on at least one surface of thehot-rolled or annealed hot-rolled sheet in a coating amount of about 0.5g/m² to about 4.0 g/m².
 11. The method according to claim 9, wherein alubricant coat comprising an acrylic resin, calcium stearate, andpolyethylene wax is coated and baked on at least one surface of thehot-rolled or annealed hot-rolled sheet in a coating amount of about 0.5g/m² to about 4.0 g/m².
 12. A fuel tank comprising a ferritic stainlesssteel sheet made from the method according to claim
 8. 13. A fuel pipecomprising a ferritic stainless steel sheet made from the methodaccording to claim 8.